Drilling fluids are used in wells drilled by the rotary method. In that process, a fluid is generally circulated downward through a hollow drill pipe, whereupon it issues through ports in the bit attached to the lower end of the drill pipe, and rises to the top of the well in the space between the drill pipe and the walls or casing of the hole. The fluid is cleaned of any cuttings or drilling debris which it has brought to the surface, and then recirculated into the drill pipe. The bore hole normally is maintained full of drilling fluid during drilling. The fluid functions to raise cuttings, to keep formation fluids from issuing into the hole by exerting a higher pressure hydrostatically, to cool and lubricate the bit and drill string, to buoy the drill pipe, and to bring formation samples to the surface for inspection. Accordingly, the fluid must have a suitable density, 2.0 g/cc or higher often being necessary; it must have proper rheological characteristics, in particular an apparent viscosity at high rates of shear of ten or twenty times that of water and a low but appreciable gel strength; however, it must be subject to thixotropic increase upon standing, and be able to resist filtration into a porous medium.
The first fluids consisted simply of clay in water, however, clays alone rarely permit fluid densities greater than about 1.25, so that finely ground, inert materials of high intrinsic density were later introduced for deeper drilling with its higher hydrostatic pressures. Barite, hematite, celestite, witherite, and other minerals have been used.
Water-base fluids are subject to increase in consistency from a number of causes, for example, an increased solids content from formation cuttings and salts and thermal degradation of the thinners. Large quantities of polyphosphates were formerly used in combatting thickening in drilling fluids, particularly from flocculation. Polyphosphates are not well suited, however, to drilling fluids used at extreme depths because the higher bottom hole temperatures rapidly bring about hydrolytic degradation to orthophosphates. Of growing importance, therefore, is the use of organic thinners, the most widely used of which are lignite and lignosulfonate derivatives, especially ferrochrome lignosulfonate. Although more stable than polyphosphates, lignite and lignosulfonate also have use limitations; i.e., they require high pH, degrade above 375.degree. F., and generally require high use levels.
Polymers have also been added to drilling fluids for improving various properties. For example, U.S. Pat. No. 3,332,872 teaches the use of a copolymer of styrene and maleic anhydride to control viscosity of drilling fluids. U.S. Pat. No. 3,730,900 employs styrene sulfonic acidmaleic anhydride copolymers as stabilizers in drilling fluids and U.S. Pat. No. 3,764,530 employs non-halogen containing acrylic acid polymers to reduce thermal degradation in these fluids.
Such polymers have also found use as scale control agents. For example, U.S. Pat. Nos. 4,065,607 and 4,223,120 teach the use of terpolymers of maleic anhydride, acrylamide and a monomer such as octene or styrene and U.S. Pat. No. 4,390,670 teaches the use of acrylate-maleate copolymers.